Remove dependence on exam class

[jlaaser]

Use of "exam" class is making it difficult to e.g. include a table of contents or an about page as part of the frontmatter; would be better to be able to use the "book" or "memoir" class, but current versions of exercises use several exam class-specific environments and commands, including questions, parts, subparts, and solutions. However, the document does NOT require many of the other features of the exam class, including point tallying, so these environments could probably be replaced by standard latex environments.

Suggested fix:

• replace "questions", "parts", and "subparts" environments with "enumerate", and replace "question", "part", and "subpart" commands with "\item"
• define custom "solution" environment (for now, just have it put in a \vspace, but should retain environment notation so that it's easy to eventually implement an option to include solutions to be shown in an instructors' version)
• replace "exam" class with "book" or "memoir" and fix any associated formatting changes